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- formal scheme
- Spf
- Witt vector
- mixed characteristic
de Rham-Witt cohomology
Motivation: take $X\in {\mathsf{sm}}{\mathsf{Alg}}{\mathsf{Var}}_{/ {k}} $, and define \({ {H}^{\scriptscriptstyle \bullet}} _\mathrm{dR}(X_{/ {k}} ) \coloneqq { {{\mathbb{H}}}^{\scriptscriptstyle \bullet}} ( { {\Omega}^{\scriptscriptstyle \bullet}} _{X/k})\) using hypercohomology. Over \({\mathbb{C}}\) this will be isomorphic to singular cohomology and take values in \({\mathsf{k}{\hbox{-}}\mathsf{Mod}}\), but if \(\operatorname{ch}k = p\) then the cohomology is entirely \(p{\hbox{-}}\)torsion. So Grothendieck/Berthelot define crystalline cohomology which takes values in \({\mathsf{W(k)}{\hbox{-}}\mathsf{Mod}}\), the \(p{\hbox{-}}\)typical Witt vectors. Originally this was defined in terms of the structure sheaf of the crystalline topos of \(X\), but a more modern definition realizes it as \({ {H}^{\scriptscriptstyle \bullet}} _{\mathrm{crys}}(X_{/ {k}} ) = { {{\mathbb{H}}}^{\scriptscriptstyle \bullet}} ( { {\Omega}^{\scriptscriptstyle \bullet}} _{X, \mathrm{drW}})\), the hypercohomology of the de Rham-Witt complex. This is a lift of algebraic de Rham in the following sense: \(\cofib(p { {\Omega}^{\scriptscriptstyle \bullet}} _{X, \mathrm{drW}} \to { {\Omega}^{\scriptscriptstyle \bullet}} _{X, \mathrm{drW}}) \simeq { {\Omega}^{\scriptscriptstyle \bullet}} _{X/k}\) is a quasi-isomorphism of cochain complexes of sheaves.